1. Field of the Invention
The present invention relates generally to a digital technique for compensating for antenna load mismatch and, more specifically, to using Digital Signal Processing (DSP) to detect, measure, and ameliorate antenna load mismatch.
2. Description of the Related Art
In radio transmitters typically used in mobile and/or portable devices, such as, e.g., mobile phones, a power amplifier (source) is connected to an antenna (load), with the amplifier designed to perform best with a nominal load impedance of (50+j0) Ω. However, in real-case scenarios, the antenna is exposed to conditions which strongly perturb its radiated electromagnetic field (typically caused by the proximity of some object), which therefore makes its actual impedance significantly diverge from the nominal value.
In order to maximize the power transfer from a source to a load in an electrical circuit, the load impedance has to equal the complex conjugate of the source impedance: when this condition is not met, the power transferred to the load is sub-optimal, therefore resulting in antenna mismatch power loss.
Due to the intrinsic limitations in both the voltage and current swing of the amplifier (caused by other than power loss), antenna load mismatch may also cause distortion of the transmitted signal. Distortion, often measured in terms of Error Vector Magnitude (EVM), causes a reduction of the Signal-to-Noise Ratio (SNR) at the receiver end, which, if severe enough, may make the received signal not intelligible anymore, eventually causing the link to fall.
The traditional approach to deal with antenna mismatch is to insert an isolator between the power amplifier and the antenna. The main objective of the isolator is to avoid the power reflected from the mismatched load to enter back into the amplifier, potentially causing it to break. Apart from the obvious increase in the cost of the application, this solution also has as a drawback a reduction of the radiated output power which equals the insertion loss of the isolator.
A more elaborate approach to overcome antenna mismatch is to insert a tunable matching network between the power amplifier and the antenna. While this is the most elegant and robust solution to maximize the output power in case of antenna load mismatch, the tunable discrete components needed to realize such a tuning network tend to be bulky, expensive, and result in non-negligible insertion losses, even when exercising a perfectly matched load.
A number of analog adaptive techniques have been developed throughout the years to automatically adjust transmit gain, typically based on power or voltage detected to go onto the load, but those require dedicated analog circuitry and typically do not allow for a programmable EVM target.
Thus, there is a need to avoid the mismatch power loss without a dedicated matching network, i.e., a need for a mechanism to detect load mismatch would allow the transmitter to reduce the amount of distortion by backing-off the transmitted signal level, in a way that EVM is kept under control, so that the receiver can compensate with higher gain to the power loss, therefore allowing the link to stay alive.